In general, with electrophotographic liquid developers, carbon black, Nigrosine, phthalocyanine blue and other organic and inorganic pigments and dyes and alkyd resins, acrylic resins, rosin, synthetic rubbers and other natural or synthetic resins are dispersed in a high resistance/low dielectric constant liquid such as a petroleum-based aliphatic hydrocarbon, and a polarity controlling agent such as a polymer containing a metal soap, lecithin, linseed oil, higher fatty acid or vinylpyrrolidone is added.
The resin was dispersed in such developers as insoluble latex particles in a particle form with a diameter of a few nanometers to a few hundred nanometers. However, with conventional liquid developers, a soluble resin for dispersion stabilization and the polarity controlling agent assume a form in which they were readily diffusible in solution since there was insufficient bonding between the soluble resin for dispersion stabilization and the polarity controlling agent and insoluble latex particles. For this reason, there are the disadvantages that long term storage and repeated use cause the soluble resin for dispersion stabilization to separate from the insoluble latex particles, the particles sediment, agglutinate and accumulate and the polarity becomes indistinct. Further, because the particles which have agglutinated and accumulated are difficult to redisperse, the particles remain adhering to the developing apparatus and stain the image portion and a breakdown of the developing apparatus, for example, by blocking the liquid feed pump occurs.
In order to remedy this disadvantage, there has been a proposal whereby the soluble resin for dispersion stabilization and the insoluble latex particles are chemically bonded is disclosed in, for example, U.S. Pat. No. 3,990,980. However, although these liquid developers have to some extent improved dispersion stability of the particles with respect to natural sedimentation, they are not yet satisfactory. Moreover, when used in actual developing devices, they have the disadvantages that they are unsatisfactory in terms of practical redispersion stability in that the toner adhering to various parts of the device hardens into a coated film and redispersion is difficult. In addition, they also become a cause of breakdown in the device, reprographic image staining and the like. Further, in the production of the resin particles described above, there are notable restrictions on the combination of the dispersion stabilizer and insolubilizing monomer which are used to produce monodisperse particles with a narrow particle size distribution. As a general rule, particles with a broad particle size distribution containing large amounts of coarse particles, or polydisperse particles in which 2 or more average particle sizes are present result. Further, it is difficult to obtain the desired average particle size with monodisperse particles with a narrow particle size distribution and large particles of 1 .mu.m or more or extremely fine particles of 0.1 .mu.m or less are formed. In addition, there are additional problems, for example, the dispersion stabilizer used must be produced by a production process which is complicated and requires a long time.
Further, in order to eliminate the abovementioned disadvantages, methods for improving the degree of dispersion, the redispersibility and the storage stability of the particles by using insoluble dispersion resin particles which are copolymers of an insolubilizing monomer and a monomer containing a long chain alkyl moiety or a monomer containing 2 or more types of polar components are disclosed in JP-A No. 60-179751 and JP-A No. 62-151868 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application"). Further, methods for improving the degree of dispersion, redispersibility and storage stability of the particles by using insoluble dispersion resin particles of copolymers of insolubilizing monomers and monomers containing long chain alkyl moieties in the presence of polymers which make use of bifunctional monomers or monomers which make use of macromolecular reactions are disclosed in JP-A No. 60-185963 and JP-A No. 61-63855.
Recently, investigations have been made into methods for printing large numbers of copies, at least 5,000 copies or more, employing an offset printing masterplate using an electrophotographic system, and improvements in masterplates have been made, in particular. As a result, it has become possible to print 10,000 copies or more with a large plate size. In addition, the shortening in the operation time for electrophotographic plate making system has also advanced and improvements in the rapidity of the developing/fixing stages are being made.
Dispersion resin particles produced in accordance with the procedures disclosed in the above-mentioned JP-A No. 60-179751, JP-A No. 62-151868, JP-A No. 60-185963 and JP-A No. 61-63855 have not necessarily resulted in a satisfactory performance in the dispersibility or redispersibility of the particles when the developing speed has been increased, nor in the printing resistance when the fixing time has been shortened or in cases involving large sized masterplates (for example, size A3 or larger).
This invention is to resolve the problems of conventional liquid developers as outlined above.